1. Field of the Invention
This invention relates to an integral multilayer analytical element for the assay of total protein contained in a liquid sample such as blood, serum and urine.
2. Description of the Prior Art
In the past, the determination of the protein content of a liquid sample by means of the biuret reaction has been carried out as a solution assay or wet chemistry technique [see, for example, Sugawara and Soejima, "TANPAKUSHITSU-NO-TEIRYOHO (Determination of Protein)," GAKKAI SYUPPAN CENTER (1977)]. Although the wet chemistry methods are useful, they often require an expensive and complicated equipment, troublesome handling, and a long time for the analysis. As an alternative to the aforementioned wet chemistry assay techniques, a variety of the so-called dry chemistry techniques and equipments therefor have recently been proposed, which make the rapid, simple and low-cost assay possible. For example, U.S. Pat. No. 4,132,528 (which corresponds to Japanese Patent Laid-Open Publication No. 101398/1979) discloses an integral multilayer analytical element for the detection of an analyte in a liquid sample, wherein the element comprises a spreading layer to distribute uniformly the sample spotted thereon and a reagent layer in which the sample is maintained and reacted with a reagent to provide a detectable change, said element being characterized by that the reagent layer contains substantially no sodium ion, an amount of base sufficient to provide a pH in excess of about 12 in said element, and an alkaline protective polymer. In making such dry chemistry analytical element, when a sodium ion is contained in a coating liquid providing the reagent layer, a phase separation is often found in the coating liquid during coating and it is difficult to provide a uniform reagent layer. Moreover, the reagent layer containing a sodium ion has disadvantage that it is highly hygroscopic and so, the element using such reagent layer tends to yellow and the performance of the element is rapidly reduced. It is, therefore, impossible to apply a conventional biuret reagent composition as it is, which can be employed in the solution method of the prior art and which contains sodium ion sources such as Rochelle salt (or potassium sodium tartarate) and sodium hydroxide, to the aforementioned integral analytical element.
The inventors of this invention have studied a stable, integral multilayer analytical element for the assay of total protein, wherein the conventional biuret reagent composition containing a sodium ion can be used. As a result, the inventors have found that such analytical element can be made by adding a specific compound to the reagent layer. This invention has been accomplished on the basis of the discovery described above.